This invention relates to gas discharge lamp ballast circuits and, more particularly, to ballast circuits for driving a plurality of gas discharge lamps.
A gas discharge lamp is an electrical device which exhibits certain special electrical characteristics. In particular, gas discharge lamps exhibit a negative impedance characteristic--once the arc of a gas discharge lamp has been struck, the current through the discharge medium increases while the voltage drop between the lamp electrodes decreases. Therefore, it is necessary to provide means for limiting the current as an element of the ballast circuit. If current-limiting means are not provided, lamp failure or transformer burnout generally results.
Because of the negative impedance characteristic, parallel operation of gas discharge lamps is generally precluded even though it provides certain desirable features. When parallel operation of gas discharge lamps is attempted, the arc in one lamp is generally struck first, resulting in a voltage drop across the parallel combination. Therefore, the first lamp struck eventually carries all of the current supplied to the parallel lamp combination, resulting in only one lamp of the parallel-connected set being started and all the rest staying dark. Obviously, such a mode of operation is not tolerable. Accordingly, series operation of gas discharge lamps has been seen as a more desirable mode of operation. However, series operation of gas discharge lamps operated at high frequency (20 kilohertz and above) may result in capacitive coupling between the lamps and surrounding ground planes. This capacitive coupling results in substantial leakage currents through the lamp's glass envelope. This phenomenon is more pronounced in series-connected lamps because the large voltage drops which occur along the lamp string create a significant potential difference between the lamps and ground.
Another disadvantage of the high voltages necessary for series operation of gas discharge lamps is the hazard posed by the voltage required to turn on all the lamps. When one of the lamps is removed from its socket the voltage at the upper socket will equal the start up voltage across the lamp string since removal of the lamp interrupts the current flow. Therefore, removal of a single lamp in a series chain results in the appearance of hazardous voltage at the lamp socket. In order to avoid the high voltage in series-connected lamp circuits it is at times necessary to employ complex, and expensive, control schemes. In a parallel connected configuration, the maximum voltage which will appear at the lamp socket is the voltage required to turn on one lamp, which is ordinarily not enough to present problems. Therefore, parallel operation would normally be preferred if a way could be found to overcome the start up problems discussed previously. This problem may be corrected by using an "isolated-series" configuration (see FIG. 6) for connecting the discharge lamps to the transformer. Such a configuration will be described in greater detail hereinafter.
The principal concern of this application is the design of an improved "isolated-series" ballast circuit for powering gas discharge lamps. It is recognized that some ballast circuit designs also incorporate a means for lamp starting. However, the discussion herein generally relates to the problem of driving lamps which have already been started. Therefore, means for starting the lamps will not be illustrated or described in detail in this application as it is assumed that one of ordinary skill in the art will be well acquainted with such means.
As described above, it is necessary to provide a current-limiting means for gas discharge lamp ballast circuits. Since resistive current limiting would degrade the operating efficiency of the ballast circuit, the current-limiting means typically comprises some form of inductance. The reactance of the inductor limits the flow of current through series-connected lamps. In addition, the typical electronic ballast circuit for gas discharge lamps also includes a transformer to step up the input voltage and isolate the voltage source from the lamps.
In view of the problems associated with both series and parallel operation of multi-lamp circuits, several arrangements have been proposed to combine the advantages of parallel operation with the startup advantages of series operation while minimizing their respective disadvantages. Most of the suggested arrangements have involved the use of complex magnetic circuits which include multiple external inductors. Several of these circuits are mentioned briefly hereinbelow and described more fully with respect to the detailed description of our invention.
One suggested gas discharge lamp ballast circuit, illustrated in FIG. 2, employs an isolation transformer with a single secondary winding. The discharge lamps are connected essentially in parallel across the secondary winding of the isolation transformer through two separate, uncoupled inductors. Each inductor separately limits current in the lamp to which it is connected. This configuration reduces leakage current but has the disadvantage of increasing the volt-ampere requirements of the ballast circuit to compensate for the inductor losses. Further, in this configuration each lamp requires a separate ballast inductor which adds to the size and cost of the ballast circuit.
Another proposed gas discharge lamp ballast circuit, illustrated in FIG. 3, also employs a transformer with a single secondary winding. The lamps are connected to one side of the secondary winding through a current-limiting inductor and a "current-sharing inductor". The smaller current-sharing inductor is designed to balance the current through the two lamps. However, the current-sharing inductor only tolerates small lamp-to-lamp voltage differences without saturation. Furthermore, although the current-sharing coil is smaller than the current-limiting inductor it replaces, it still contributes substantially to the size, weight and cost of the circuit. In addition, the circuit of FIG. 3 also requires more total volt-amperes than conventional series lamp operation to overcome losses in the inductor and the current-sharing coil. Finally, it would be difficult to extend this current-sharing coil concept to more than two lamps.
A third solution to the gas discharge lamp ballast circuit problem which is particuarly relevant to this invention utilizes a transformer connected in "isolated-series" configuration. Such a transformer is illustrated in FIG. 4. An isolated-series ballast circuit for driving a plurality of gas discharge lamps comprises a multi-legged transformer core with at least three legs. A primary winding and at least two secondary windings are disposed on separate transformer legs. Gas discharge lamps are connected across each of the secondary windings. In addition, isolated-series connected transformers preferably include means, such as secondary taps, to heat the gas discharge lamp filament electrodes. The isolated-series ballast circuit is readily extendable to circuits in which three, four or more gas discharge lamps are driven simultaneously by providing an additional transformer leg and corresponding secondary winding for each additional lamp. Because of the magnetic characteristics of the circuit described more fully hereinbelow, isolated-series operation achieves substantially all of the advantages of both series and parallel operation with few of their respective disadvantages. Isolated-series operation is therefore a desirable method of driving multiple gas discharge lamps.
The design of isolated-series ballast circuits is complicated by a number of factors. First, ballast transformers normally step up the input signal to provide sufficient voltage at the secondary to drive the gas discharge lamp. In order to achieve the maximum efficiency in such a step-up transformer, it is desirable to minimize the number of windings in both the secondary and the primary to reduce the winding losses. However, if the number of primary windings is too small, the transformer core will saturate, limiting the output voltage of the secondary, and the core losses will increase. Therefore, it is necessary to include sufficient primary turns to avoid saturating the transformer within the desired range of input voltages.
A second factor complicating the design of "isolated-series" and other, more conventional ballasts, is the inherent leakage inductance of the transformer. Physical transformers are normally modeled as ideal transformers with parallel magnetizing and series leakage inductances. Because of the difficulties encountered in attempting to quantize leakage inductance in conventional ballast transformers, it has been desirable to limit the effect of the leakage inductance by coupling the primary and secondary together as closely as possible. The closest possible coupling occurs when the primary and secondary are wound on the same core leg. However, winding the primary and secondary on the same core leg presents significant manufacturing and electrical drawbacks in circuits where more than one secondary is necessary. Mechanically, it is extremely difficult to wind multiple secondaries on the same leg as the primary. Electrically, the amount of leakage flux increases with every additional winding separating a particular secondary from the primary, resulting in unpredictable and unbalanced secondary voltages.
Finally, as will be readily apparent from the description hereinbelow, the output currents of isolated-series connected transformers are inherently limited by the leakage inductance described previously. However, this leakage inductance has not heretofore been considered readily controllable. Therefore, in certain applications, a current-limiting inductance has been included in the output of the isolated-series transformer. The current-limiting inductance of an isolated-series transformer may be a separate circuit component or it may be integrated into the transformer structure. One method of integrating the ballast inductor into the transformer structure is to utilize a "gapped-leg" configuration. The operation of such a "gapped-leg" transformer is described in greater detail herein with reference to FIG. 5.
Both the gapped leg and the external inductor have disadvantages. The external ballast inductor is a bulky, expensive additional element, and is especially disadvantageous when it is necessary to provide a separate inductor for each lamp. The gapped leg adds complexity and expense to the transformer manufacture while proving to be a less than ideal ballasting inductor. Therefore, it would be advantageous to provide output inductance for an isolated series connected transformer for multilamp ballast circuits which does not share the inherent disadvantages of the gapped-leg transformer or the external inductor configurations.
Recent advances in magnetics have made it possible to quantize the leakage characteristics of transformers and to rely upon leakage phenomena as a design parameter rather than a parasitic parameter of the transformer. The value of the leakage inductance can be calculated from the geometry of the transformer core and winding using equations such as those derived for several simple cases by A. Daujahre in "Modeling and Estimation of Leakage Phenomena in Magnetic Circuits", PhD. Thesis, California Institute of Technology, Pasadena, CA. 1986, which is hereby explicitly incorporated by reference.